Calorimeters measure heat flows associated with chemical and/or physical reactions and conversions. They serve to determine thermodynamic and kinetic quantities of a reaction medium. Reaction media that can be used include pure substances or mixtures of substances which are in most cases dissolved in a suitable solvent. The reaction medium can be liquid or gaseous.
Reaction calorimeters can further be used to obtain data that are relevant for the safety of chemical and/or physical reactions, or to establish data of the kind required for example for a so-called scale-up process, i.e. for reproducing a small-volume laboratory reaction in a large-volume process system.
The reaction calorimeter can comprise a reactor or a suitable container, for receiving the reaction medium, that is at least in part encased in a reactor jacket. The temperature of the reaction medium can be controlled in different ways.
The reaction calorimeters of the state of the art are in many cases operated according to the heat flow principle or according to the principle of power compensation.
Under the power compensation principle, which is in most cases performed under isothermal conditions, a compensation heater is arranged in the reactor of the calorimeter. The compensation heater can act directly on the reaction medium and is used to control the temperature in the reactor and/or the temperature of the reaction medium. By means of an associated temperature control unit the temperature of the reactor jacket is kept essentially constant at a level below the reaction temperature.
At the beginning of a measurement, a certain amount of heating power is delivered to the reaction medium by the compensation heater. In the course of the reaction, the delivery of heating power is increased or decreased, depending on whether the reaction is an endothermic or exothermic reaction.
Due to the direct delivery of heat, temperature fluctuations in the reactor can be compensated rapidly. In addition, the analysis of the results is relatively simple, as the delivered heat amount can be determined directly.
However, the compensation heater is only capable of heating, but not of cooling, so that the minimum power delivery to the reactor is around zero watts. This has the consequence that the compensation heater has to permanently deliver energy to the system at a rate corresponding to the maximum heat rate to be measured for an exothermy, i.e. that the reaction generates no heat. The maximum heat rate is not always accurately known at the beginning of an experiment and can only be estimated. For reasons of safety, experiments are therefore conducted mostly with an assumed maximum heat rate that is set too high, which has the consequence that heat is delivered to the reactor by the compensation heater at a quite considerable rate during the entire experiment and subsequently needs to be carried away through the reactor wall.
Due to the high rate at which heat is brought in, the reaction medium can locally overheat in the vicinity of the compensation heater. This can have a negative effect on the course of the reaction as well as on the safety of a reaction taking place in the reactor. In addition, the measurement results exhibit a strong dependency on changes in the heat transfer conditions and thus on the global heat transfer coefficient, wherein the latter is subject to a change over time if the heat flow changes during the experiment for example because of a change of the viscosity of the reaction medium. A change can likewise occur in the heat exchange surface due to chemical or physical processes, as a result of adding a substance or taking a sample, whereby the global heat transfer coefficient can also be affected.
Under the heat flow principle, the temperature in the reactor is controlled by the temperature of the reactor jacket. The latter is in most cases connected to a temperature control unit which is capable of cooling as well as heating. This principle has the advantage that the reactor jacket provides a relatively large heat exchange surface with the reactor, so that conditions of local overheating in the reaction medium can essentially be avoided. However, the temperature control is relatively sluggish in comparison to power compensation, and the heat capacity of the reactor jacket needs to be taken into account in the process of determining the quantities to be measured.
A reaction calorimeter with a reactor jacket and a compensation heater is disclosed in DE 44 05 267 A1, wherein the reactor jacket has an essentially constant temperature over time and the compensation heater keeps the reactor at a constant temperature, responding with a small time constant. For adiabatic measurements, the temperature of the reactor jacket can be made to follow the temperature inside the reactor, the latter being equipped for this purpose with a supplemental heater. This reaction calorimeter represents a combination of the two principles described above.
It is therefore an object to provide an improved method for the safe operation of a calorimeter, as well as to provide a suitable calorimeter, wherein the advantages of the power compensation principle and the heat flow principle are advantageously combined and an automatic tracking of the reactor jacket temperature is possible.